U.S. patent number 7,358,899 [Application Number 10/070,107] was granted by the patent office on 2008-04-15 for distance estimation in a communication system.
This patent grant is currently assigned to Nokia Corporation. Invention is credited to Marko Alanen, Gudni Gunnarsson, Timo Rantalainen, Ruutu Ville.
United States Patent |
7,358,899 |
Ville , et al. |
April 15, 2008 |
Distance estimation in a communication system
Abstract
The present invention relates to estimation of the distance
between stations that communicate in a communication system over a
radio interface. In the system at least one of the stations
transmits signal bursts in time slots in accordance with a timing
structure and at least one station receives the signal bursts. The
method comprises determination of a first timing of a signal burst
received at the receiving station, the first timing being
associated with the first component of the received signal burst
that meets a predefined condition. A second timing of the received
signal burst is also determined for use in adjustment of internal
timing of the receiving station for receiving and/or transmitting
further signal bursts from and/or to the transmitting station. A
timing offset is also determined between the stations. The distance
between the stations is estimated based on the timing offset and
the first timing of the received signal burst.
Inventors: |
Ville; Ruutu (Espoo,
FI), Rantalainen; Timo (Helsinki, FI),
Alanen; Marko (Tempere, FI), Gunnarsson; Gudni
(Tempere, FI) |
Assignee: |
Nokia Corporation (Espoo,
FI)
|
Family
ID: |
39281607 |
Appl.
No.: |
10/070,107 |
Filed: |
August 31, 2000 |
PCT
Filed: |
August 31, 2000 |
PCT No.: |
PCT/EP00/08551 |
371(c)(1),(2),(4) Date: |
July 23, 2002 |
PCT
Pub. No.: |
WO01/19112 |
PCT
Pub. Date: |
March 15, 2001 |
Current U.S.
Class: |
342/458;
342/118 |
Current CPC
Class: |
G01S
11/02 (20130101) |
Current International
Class: |
G01S
3/02 (20060101) |
Field of
Search: |
;342/118,129,133,458
;459/450,456.1,456.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 767 594 |
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Apr 1997 |
|
EP |
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0 917 385 |
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May 1999 |
|
EP |
|
07-181242 |
|
Jul 1995 |
|
JP |
|
08-079170 |
|
Mar 1996 |
|
JP |
|
WO 97/23785 |
|
Jul 1997 |
|
WO |
|
WO99/06851 |
|
Feb 1999 |
|
WO |
|
WO 99/27738 |
|
Jun 1999 |
|
WO |
|
Primary Examiner: Phan; Dao
Attorney, Agent or Firm: Lackenbach Siegel LLP Bauer; Robert
M.
Claims
The invention claimed is:
1. A method: determining a first timing of a signal burst received
from a transmitting station at a receiving station, the first
timing being associated with a first component of the received
signal burst that meets a predefined condition; determining a
second timing of the received signal burst, said second timing
being for use in adjustment of internal timing of the receiving
station for communicating further signal bursts with the
transmitting station; determining timing delay information for the
communication between the transmitting and receiving stations; and
estimating a distance between the transmitting and receiving
stations based on said timing delay information and information
about the first timing of the received signal burst.
2. A method according to claim 1, comprising determining a
difference between the first timing and the second timing, wherein
the determined difference is used as a correction value in the
estimation of the distance between the stations.
3. A method according to claim 1, wherein a timing advance value
that is based on the timing delay information is used in said
estimation of the distance, comprising: determining a difference
between the first timing and the second timing; and subtracting the
difference between the first timing and the second timing from the
timing device value.
4. A method according to claim 1, wherein the respective timings
are determined using an impulse response of the received signal
burst.
5. A method according to claim 4, wherein the determination of the
second timing is based on the mass center of the impulse
response.
6. A method according to claim 2, wherein the receiving station
determines the difference between the first timing and the second
timing and the first and second timings are determined using an
impulse response of the received signal burst.
7. A method according to claim 6, comprising transmitting the
determined difference between the first timing and the second
timing to the transmitting station for processing.
8. A method according to claim 1, comprising: use of different
reception and transmission timings at the receiving station,
wherein the reception timing of the receiving station is adjusted
in accordance with the determined second timing and the
transmission timing for transmitting a response signal from the
receiving station to the transmitting station is adjusted in
accordance with the determined first timing; receiving the response
signal at the transmitting station; determining at the transmitting
station a timing of the received response signal such that the
timing is based on a component of the received response signal
corresponding said first component; and determining a difference
between the transmission timing and the timing of the received
response signal.
9. A method according to claim 8, wherein the receiving station
informs the communication system that it uses different reception
and transmission timings.
10. A method according to claim 1, wherein the receiving station
comprises a mobile station of a cellular communication system and
the transmitting station comprises a base station of the cellular
communication system.
11. A method according to claim 1, wherein the receiving station
comprises a base station of a cellular communication system and the
transmitting station comprises a mobile station of the cellular
communication system.
12. A method according to claim 11, wherein the difference between
the timings is subtracted from the timing advance by the base
station.
13. A method according to claim 1, wherein the predefined condition
is met by the signal component of the signal burst that arrives as
a first detectable component of the transmission.
14. A method according to claim 1, wherein the predefined condition
comprises a threshold value for the components.
15. A method according to claim 1, wherein one of the stations is a
mobile station of a cellular communication system and at least one
of the stations is a fixedly positioned base station, further
comprising a step of determining the current geographical location
of the mobile station by means of the distance between the mobile
station and said at least one base station.
16. A method according to claim 15, further comprising step of:
determining at least-one further distance between the mobile
station and at least one further base station; and combining the
results of the at least two determinations for estimating the
current geographical location of the mobile station.
17. A method according to claim 1, wherein one of the stations is a
mobile station, comprising step of combining the result of the
estimation of the distance between the mobile station and another
station with at least one further result obtained from another
determination relating to the location of the mobile station.
18. A method according to claim 1, comprising communicating
information of at least one of the accomplished determinations to a
location service node of the communication system.
19. A communication system comprising: a transmitting station
arranged to transmit one or more signal bursts over time slots in
accordance with a timing structure of the communication system; a
receiving station configured to; receive the one or more signal
bursts, determine a first timing of a signal burst received at the
receiving station, the first timing being associated with a first
component of the received signal that meets a predefined condition,
determine a second timing of the received signal burst for use in
adjustment of the receiving station for receiving and/or
transmitting further signal bursts, determine timing delay
information for communication between the stations, and estimate a
distance between the transmitting and receiving stations based on
the timing delay information and information about the first
timing.
20. A communication system according to claim 19, wherein the
receiving station is configured to determine a difference between
the first timing and the second timing and is correct an initial
estimate of the distance between the stations on the basis of the
determined difference.
21. A communication system according to claim 19, wherein the
receiving station is configured to base the distance estimation on
a timing advance value derived from the timing delay information,
to determine a difference between the first timing and the second
timing, and to subtract the difference between the first timing and
the second timing from the timing advance value.
22. A communication system according to claim 19, wherein the
receiving station is configured to make use of an impulse response
of the received signal burst.
23. A communication system according to claim 22, wherein the
determination of the second timing is based on the mass center of
the impulse response and the determination of the first timing is
based on a first component of the signal to arrive.
24. A communication system according to claim 19, wherein the
receiving station is configured to use different timings for
reception and transmission.
25. A communication system according to claim 19, wherein the
receiving station comprises a mobile station of a cellular
communication system and the transmitting station comprises a base
station of the cellular communication system.
26. A communication system according to claim 19, wherein the
receiving station comprises a base station of a cellular
communication system and the transmitting station comprises a
mobile station of the cellular communication system.
27. A communication system according to claim 19, wherein one of
the stations is a mobile station of a cellular communication system
and at least one other of the stations is a fixedly positioned base
station, the system configured to determine the current
geographical location of the mobile station by means of the
distance between the mobile station and said at least one base
station.
28. A communication system according to claim 19, comprising
further a location service node for providing geographical location
information.
29. A communication system comprising: a transmitting station
comprising means for transmitting one or more signal bursts over
time slots in accordance with a timing structure of the
communication system; a receiving station comprising: means for
receiving the one or more signal bursts; control means for
determining a first timing of a signal burst received at the
receiving station, the first timing being associated with a first
component of the received signal that meets a predefined condition;
control means for determining a second timing of the received
signal burst for use in adjustment of the receiving station for
receiving and/or transmitting further signal bursts; control means
for determining timing delay information for communication between
the stations; and control means for estimating a distance between
the transmitting and receiving stations based on the timing delay
information and information about the first timing.
30. A station in a communication station comprising a processing
unit configured to: determine a first timing of a received signal
burst received from a transmitting station at the station, the
first timing being associated with a first component of the
received signal burst that meets a predefined condition, determine
a second timing of the received signal burst, said second timing
being for use in adjustment of internal timing of the station for
communicating further signal bursts with the transmitting station,
determine timing delay information for the communication between
the transmitting station and the station, and estimate a distance
between the transmitting station and the station based on said
timing delay information and information about the first timing of
the received signal burst.
31. A station in a communications system comprising means for
determining a first timing of a received signal burst received from
a transmitting station at the station, the first timing being
associated with a first component of the received signal burst that
meets a predefined condition; means for determining a second timing
of the received signal burst, said second timing being for use in
adjustment of internal timing of the station for communicating
further signal bursts with the transmitting station; means for
determining timing delay information for the communication between
the transmitting station and the station; and means for estimating
a distance between the transmitting station and the station based
on said timing delay information and information about the first
timing of the received signal burst.
32. A method comprising: receiving at a first station a signal
burst transmitted from a second station; determining first timing
information associated with the signal burst; determining second
timing information associated with the signal burst; determining
timing delay information associated said second timing of the
signal burst; and estimating a distance between the first and
second stations based on said timing delay information and said
first and second timing information.
33. The method if claim 32 comprising calculating a difference
between said first and second timing information, wherein said
estimating uses said difference.
34. A computer-readable medium having computer-executable
components for: determining a first timing of a received signal
burst received from a transmitting station at a receiving station,
the first timing being associated with a first component of the
received signal burst that meets a predefined condition;
determining a second timing of the received signal burst, said
second timing being for use in adjustment of internal timing of the
receiving station for communicating further signal bursts with the
transmitting station; determining timing delay information for the
communication between the transmitting and receiving stations; and
estimating a distance between the transmitting and receiving
stations based on said timing delay information and information
about the first timing of the received signal burst.
35. A computer-readable medium having computer-executable
components for: receiving at a first station a signal burst
transmitted from a second station; determining first timing
information associated with the signal burst; determining second
timing information associated with the signal burst; determining
timing delay information associated said second timing of the
signal burst; and estimating a distance between the first and
second stations based on said timing delay information and said
first and second timing information.
Description
FIELD OF THE INVENTION
The present invention relates to estimation of a distance between
stations in a communication system, and in particular, but not
exclusively, to use of at least one timing parameter in the
distance estimation.
BACKGROUND OF THE INVENTION
A cellular communications system is based around cells or similar
radio coverage areas. Examples of cellular telecommunications
systems include standards such as, without limiting to these, GSM
(Global System for Mobile communications) or various GSM based
systems (such as GPRS: General Packet Radio Service), AMPS
(American Mobile Phone System), DAMPS (Digital AMPS), CDMA (Code
Division Multiple Access) or the proposed WCDMA (Wideband CDMA), or
UMTS (Universal Mobile Telecommunications System) or other third
generation communication system and so on. In general, a cell
coverage area of a telecommunication system can be defined as a
certain geographically limited area covered by one or several base
transceiver stations (BTS) serving user equipment (UE) or mobile
stations (MS) via an air or radio interface and usually connected
to a base station subsystem (BSS). Several cells connected to each
other cover a larger geographical area, and thus several cells form
together a larger geographical area, and cellular telecommunication
network.
Each coverage area of the system can be controlled by an
appropriate controller apparatus. The controller can be connected
further to a gateway or linking apparatus, such as a gateway GPRS
support node (GGSN) or gateway mobile switching center (GSMC),
linking the cell to the other communication networks, such as to a
PSTN (Public Switched telecommunications Network) or to a data
network, such as to a X.25 based network or to a TCP/IP
(Transmission Control Protocol/Internet Protocol) based
network.
The user equipment (UE) (that will be referred to in the following
as mobile station or MS) within one of the cells of the
telecommunications systems is respectively controlled by the
controller functionality of the cell. The MS may be controlled by
only one controller at time. However, the MS may also be
simultaneously controlled by several controllers. This may occur
e.g. when the cells overlap or in so called soft handoff mode,
where the MS may be in communication with two base stations and
those base stations may be connected to different controllers, or
when one controller is controlling another controller controlling
the MS. One controller can be defined as the serving (main)
controller whereas the others act as secondary controllers.
The mobile station communicates with an appropriate network
controller and provides the controller with various types of
information. For instance, when the mobile station changes from one
cell to a new cell or when the mobile station is switched on in one
of the cells after having been switched off or unreachable for a
while, the mobile station will send a message containing a MS
identifier (ID) to the controller of the new cell. The controller
of the new cell may be the same as in the previous cell or it may
be different from the previous controller handling the MS before
the cell change. The MS ID comprises data required by the
telecommunications system in order to be able to handle the
incoming/outgoing call and signalling to/from the MS, respectively.
Once the MS ID is received by the system, the system is aware of
the current cell of the particular mobile station.
In time division multiple access (TDMA) networks, a plurality of
time slots is used for the transmission between the mobile stations
and the base transceiver stations. A frame is made up of a
predetermined number of time slots. Each mobile station will be
allocated a given time slot in each frame in which to communicate
with a base transceiver station. In order to process correctly the
signal received from various mobile stations, the signal from each
mobile station must be received within its allocated time slot at
the base station. To ensure that the signals are received within
the allocated time slots the base station will provide the mobile
station with timing advance (TA) information. The timing advance
information indicates when the mobile station should transmit its
signal to the base station. The timing advance information is
required since propagation of radio waves between the transmitting
and receiving stations causes a delay in the transmission between
the stations. In other words, the travel of the radio waves from
the BTS to the MS and vice versa takes some time, and thus there is
a certain offset between the timings of the slots used by the MS
and the BTS. In order to address the delay, and more precisely, to
avoid any overlapping time slots, it is therefore necessary to
determine the time delay between the mobile station and the serving
base transceiver station.
According to an approach a mobile station in an idle mode adjusts
its internal timing according to bursts received from the serving
base station. When the MS transmits for the first time to the BTS
(access bursts), this is done by using the internal timing of the
mobile station which is delayed compared to BTS's timing. When the
BTS receives these signals, it measures the time offset between its
frame structure and the received MS bursts. The difference between
these two timings equals to the propagation delay of the radio
waves travelling from the BTS to the MS and back, i.e. twice the
distance between the stations. To avoid overlapping time slots in
the BTS, the BTS provides the MS with timing advance (TA) value
information indicating the time how much earlier the MS should
transmit towards the BTS. By means of this it is possible for the
MS to adjust its transmission so that the transmission can be
received by the BTS in slots according to the frame structure used
by the receiving BTS. During a dedicated connection the BTS
continuously determines the appropriate TA value and signals the TA
value to the MS.
One possible way to determine the timing is based on impulse
responses. The impulses response can be obtained by correlating a
known bit pattern in a burst (a so called training sequence) with
the received signal. The impulse response is a curve with the time
on X-axis (component taps) and intensity on the Y-axis (see FIG. 3
showing an exemplifying impulse response). For normal operation of
a TDMA system, such as the GSM, it is important that the reception
is adjusted so that a maximum energy will be received. Thus the
timing has been determined in the prior art from the mass center of
the impulse response.
The functionalities of the cellular communication system facilitate
a provision of an at least rough location estimate concerning the
current location of an individual mobile station. More
particularly, the cellular communication system is always aware (at
least roughly) of the current location area of such mobile stations
which are communicating with at least one of the base stations of
the system. This information is available even when the mobile
station is located within a coverage area of a visited or "foreign"
network, as the visited network is capable of transmitting the
location information of the mobile station back to the home
network, e.g. for the purposes of routing and charging.
A location service feature provided by means of a cellular
telecommunications network has been proposed. The proposed location
service can provide the last known location of a mobile station
together with a time-stamp. The proposed service functionality can
be provided by a separate network element or server which receives
the information from the various controllers of the system.
One possibility of making the location determination more accurate
is to utilize the timing delay or offset information between a
mobile station and a base station. This is based on the realization
that when the mobile station is relatively far from the base
station, the signal will travel for a longer time to the base
station as compared to when the mobile station is relatively close
to the base station. Therefore, in order to address the offset, a
mobile station relatively far from the base station will have to
send its signal to the base station more "in advance" relative to
the timing structure than a mobile station which is relatively
close to the base station. The timing advance information (TA) is
thus also a measure of the distance between the mobile station and
the base station. The timing advance information does not provide
indication as to the direction from which the signal has been
received. Thus the use of the timing advance information may
require use of some other supporting information so that the
direction can be estimated. For instance, the location may be
accomplished such that the mobile station communicates also with at
least one other and preferably with three neighboring base stations
covering the area in which the mobile station is currently located
in the event that no other indication of the direction is
available. Directional antennae or Angle of Arrival (AOA)
measurement from one base station site may also be used together
with the timing advance information. In the latter case the
location is determined to the intersection of AOA line and timing
advance circle.
Currently the timing advance is determined e.g. in the GSM system
with an accuracy of 1 bit. This corresponds to 3.69 .mu.s in time
and 1.1 km in distance. This accuracy is enough for the normal
operation of the cellular communication system. However, it may not
be enough for some special purposes, like for estimating a distance
between two or several stations of the communication system and for
geographically locating a mobile station based on the timing
advance information.
SUMMARY OF THE INVENTION
It is an aim of the embodiments of the present invention to address
one or several of the above problems.
According to one aspect of the present invention, there is provided
a method for estimating a distance between stations communicating
in a communication system over a radio interface, wherein at least
one of the stations transmits signal bursts in time slots in
accordance with a timing structure and at least one station
receives the signal bursts, said method comprising the steps:
determining a first timing of a signal burst received at the
receiving station, the first timing being associated with the first
component of the received signal burst that meets a predefined
condition; determining a second timing of the received signal burst
for use in adjustment of internal timing of the receiving station
for receiving and/or transmitting further signal bursts from and/or
to the transmitting station; determining a timing offset between
the stations; and estimating the distance between the stations
based on the timing offset and by utilizing the first timing of the
received signal burst.
In a preferred embodiment the step of estimating the distance
comprises determining a difference between the first timing and the
second timing and correcting an estimate of the distance between
the stations based on the determined difference. A timing advance
value based on the offset may be used in said estimation of the
distance, wherein a difference between the first timing and the
second timing is determined and the difference between the first
timing and the second timing is subtracted from the timing advance
value.
According to one possibility the receiving station may use
different reception and transmission timings, wherein the reception
timing of the receiving station is adjusted in accordance with the
determined second timing and the transmission timing for
transmitting a response signal from the receiving station to the
transmitting station is adjusted in accordance with the determined
first timing. The transmitting station may determine a timing of a
received response signal such that the timing is based on a
component of the received response signal corresponding to said
first component, whereafter a difference between the transmission
timing and the timing of the received response signal is
determined.
One of the stations may be a mobile station of a cellular
communication system and at least one of the stations may be a
fixedly positioned base station. The current geographical location
of the mobile station may be determined by means of the estimated
distance between the mobile station and said at least one base
station. Information of at least one of the determinations may be
communicated to a location service node of the communication
system.
According to another aspect of the present invention there is
provided a communication system comprising: a transmitting station
arranged to transmit signal bursts over time slots in accordance
with a timing structure of the communication system; a receiving
station arranged to receive the signal bursts; control means for
determining a first timing of a signal burst received at the
receiving station, the first timing being associated with the first
component of the received signal that meets a predefined condition;
control means for determining a second timing of the received
signal bursts for use in adjustment of the receiving station for
receiving and/or transmitting further signal bursts; control means
for determining a timing offset between the transmitting station
and the receiving station; and control means for estimating a
distance between the stations based on the timing offset and the
first timing.
Embodiments of the invention may provide several advantages. The
accuracy of the distance estimation between the mobile station and
the serving base station is improved, which leads to improved
location accuracy, as the timing information used for the distance
calculations is corrected by information concerning the timing for
the shortest path for radio propagation. The timing information is
already used by the communication systems and therefore some
embodiments of the invention do not require any changes to the
existing mobile stations. In addition, some embodiments of the
invention do not require any hardware changes to the existing
mobile stations or the base stations. The provided accuracy enables
embodiments where only one base station is needed for providing the
geographical location of a mobile station.
BRIEF DESCRIPTION OF DRAWINGS
For better understanding of the present invention, reference will
now be made by way of example to the accompanying drawings in
which:
FIG. 1 shows one possible network environment in which an
embodiment of the present invention may be implemented;
FIG. 2 shows an embodiment using three base stations for location a
mobile stations;
FIG. 3 shows an impulse response;
FIGS. 4A, 4B and 4C show block diagrams for possible embodiments;
and
FIG. 5 is a flowchart illustrating the operation of an embodiment
of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Reference will first be made in FIG. 1 illustrating a general view
of one possible cellular telecommunications system having a
location service. It is noted that even though the exemplifying
telecommunications network shown and described in more detail in
the following uses the terminology of a circuit switched GSM
(Global System for Mobile communications) public land mobile
network (PLMN), the proposed solution may be used in any
communication system providing some degree of location
functionality for at least one of stations communication with the
system using time slots. It should also be appreciated that even
though FIG. 1 shows three base station groups which each provide
three omnidirectional radio coverage areas 1, 2 and 3, the
embodiments of the invention may be implemented using any number of
radio coverage areas. The coverage areas 1, 2 and 3 may also be,
instead of base station coverage areas, three cell coverage areas
of the mobile communications network, where the coverage area of
one cell includes more than one base station. It is also possible
to group several cells such that one coverage area comprises more
than one cell. For example, URA (UMTS Terrestrial Radio Access
Network Registration Area) in the UMTS standard consist of a group
of cells. It is also noted that the radio coverage area may consist
of a sector of a base station provided with a directional or sector
antenna (not shown). The sector base station may use e.g. three
120.degree. directional antennas whereby three radio coverage areas
are provided, or four 90.degree. directional antennas providing
four radio coverage areas and so on, or any combinations of
different radio coverage beam widths. It should also be appreciated
that base stations may sometimes be referred to as node B (e.g. in
the UMTS standard).
In FIG. 1 each radio coverage area 1, 2 and 3 is served by the
respective base transceiver station (BTS) 4, 5 and 6. Each base
transceiver station BTS is arranged to transmit signals to and
receive signals from the mobile station (MS) 7 in the cell.
Likewise, the mobile station 7 is able to transmit signals to and
receive signals from the respective base transceiver station. The
mobile station 7 accomplishes this via wireless or radio
communication with the base stations. Typically a number of mobile
stations will be in communication with each base station although
only one mobile station is shown in FIG. 1 for clarity.
Each of the base stations is connected to a respective network
controller, which in one form of the exemplifying GSM system
comprises a base station controller 8 connected further to a Mobile
Switching Center (MSC) 9. However, as is also shown by FIG. 1, in
some arrangements the base station controller 8 controlling one or
several base stations between the network controller and the base
stations may be omitted. Each of the network controllers controls
its service area, i.e. the cells and base stations connected to it,
either directly or via the base station controller. It is noted
that more than one base station or base station group may be
connected to each network controller. Typically more than two
network controllers are also provided in a network. The network
controllers is connected to other elements or parts of the
telecommunications network system via a suitable linking or gateway
apparatus, such as Gateway Mobile Switching Center (GMSC) 14.
The implementation of the communication between the mobile station
and the controller is known, and will thus not be discussed in more
detail herein. It is sufficient to note that the interface may
comprise channels in both uplink and downlink directions between
the mobile station in the cell associated with a given base station
and that the information sent to the mobile station and the data
sent from the mobile station may be in packet form (e.g. in the
GPRS or UMTS systems). However, other formats for transmitting
information are not excluded and in alternative embodiments of the
invention. The data may thus be sent in any suitable format. The
messages sent from the mobile stations may include information
identifying the mobile station (for instance, MS ID and/or IMSI
(Mobile Station Identity and/or International Mobile Subscriber
Identity, respectively)).
FIG. 1 also illustrates the possibility of using a packet switched
mobile telecommunications service for communication with a data
network. The illustrated GPRS (General Packet Radio Service) is
based on the GSM technology and it may use the existing GSM base
stations and other network elements for its operation. The GPRS
comprises a serving GPRS support node (SGSN) corresponding
substantially to the MSC of the GSM networks for controlling the
service area and a gateway GPRS support node (GGSN) for interfacing
with the data network, for example the TCP/IP (Transport Control
Protocol/Internet Protocol) Internet. In the GPRS system the
network controller SGSN is arranged to pass on data packets to be
transmitted to the mobile station 7 by the respective base station.
The controller will also receive from the base station packets of
data which is has received from the mobile station.
The mobile station 7 is able to move within a radio coverage area
and also from one radio coverage area to another coverage area,
e.g. from cell 1 to cell 2. The location of the mobile station 7
may thus vary in time. To ensure proper operation of the system and
to avoid disconnecting a possibly ongoing call, handover procedures
which are known are provided so that all necessary information will
become available for the parties involved (e.g. for the previous
and new network controller). The mobile station may also be in
communication with more than one base station, e.g. during a so
called soft handoff procedure. The roaming mobile station 7
communicating with different base stations of the communications
system may be registered to a home location register (HLR) 13 and
also to a visitor location register (VLR) 11. It is noted that a
mobile telecommunications network may include several home location
registers and that the home location registers may be included in
one or several of the network controllers.
FIG. 1 shows also a Location Services (LCS) node 16 providing
location services for different applications or clients 18 who are
entitled to receive at least some degree of information concerning
the geographical location (or location history) of a mobile
station. The clients may be external, i.e. clients from outside the
communication system. The clients may also be internal, i.e.
functionalities or elements of the communication system itself may
use geographical location information provided by the LCS node 16
for the operation of the system (for instance, resource allocation,
location assisted handover, fleet management or location related
billing). A more detailed proposal for a location service
functionality can be found from ETSI (European telecommunications
Standards Institute) technical specification GSM 03.71. In general
terms, the LCS functionality can be defined as a feature capable of
providing information concerning the geographical location of a
particular mobile station. The geographical location of the mobile
station may be defined on the basis of the position of the mobile
station relative to the base station(s) of the mobile
telecommunications system. The location server node 16 is arranged
to receive predefined data concerning the geographical location of
the mobile station 7 and to process this data and/or some other
predefined parameters and/or to compute appropriate calculations
for determining and outputting the geographical location of the
given mobile station. The location server 16 may also be arranged
to check the client's 18 right to receive any information
therefrom.
The location server 16 has a communication connection with one or
several of the elements of the communication system, such as the
home location register 13, the visitor location register 11, 12,
the MSCs 9 and 10 or the BSC 8. The communication connection may be
implemented e.g. by means of a MAP (Mobile Application Part)
connection. It is noted that even though the LCS node 16 is shown
in FIG. 1 as a stand-alone element, it could also be integrated in
an appropriate network element, e.g. be an internal component or a
functionality of any of the controllers 8, 9, 10 or gateways of the
communication system.
The location information may be transmitted from the communication
network to the location server 16 in response to a request by the
location server. The location server 16 may also receive location
information updates periodically from the communication network.
The network may also provide the location server automatically with
updated location information in the event that the location of the
mobile station changes. The location information may be provided
with a timestamp. The timestamp is attached to the data e.g. by the
visitor location register or the home location register and
indicates the time the location data was provided.
The determination of the location of a mobile station may be based
on measurements of at least one feature of the components of a
radio signal from which it is possible to determine the distance
between the transmitting station and the receiving station. In a
TDMA system a feature which may be used for this is the timing of
the components in a time slot, as will be explained in more detail
later in this specification.
It is noted that the distance measurements may be accomplished in
the uplink and/or in the downlink, i.e. at the base station end or
at the mobile station end or both. In the case where the mobile
station is used for the determining at least a part of the
information used for location estimation purposes, it may use the
radio network for messaging the results to an appropriate network
element. The necessary location calculations and determinations
based on various collected/defined data may be accomplished at the
base station and/or the mobile station or then at an appropriate
network element having an access to all required data.
FIG. 2 illustrates a situation in which the geographical location
of a mobile station 7 is estimated by using three base stations 5,
6 and 7. The distance between the mobile station (MS) 7 and each of
the three base stations may be estimated using timing advance (TA)
information. In theory this provides three circles that are in most
cases enough to find an accurate enough estimate for the location
of the MS 7, the geographical location being the intersection point
of the three circles. In the absence of any measurement error, the
intersection of the three circles in the FIG. 2 would then
unambiguously determine the accurate location of the mobile station
7.
However, as may also be seen from FIG. 2, in practice the situation
will not be this straightforward due to various measurement errors,
such as the already mentioned insufficient accuracy of the timing
advance information for the distance measurement purposes and/or
blocking elements or obstacles in the radio propagation path (Non
Line of Sight (NLOS) conditions). The obstacles could be, for
example, buildings and other large constructions, pillars, statues,
trees, rocks, cars or similar elements between the base station and
the mobile station. The amount of measurement error depends also on
the actual distance between the transmitting station and the
receiving station. In multipath connections the possibility for
determination errors increases further.
It is noted that the three base stations of FIG. 2 are an example
only and that when implementing the embodiments of the invention
the number of base stations may be different from this example. The
improved accuracy provided by using timing information in
accordance with the following embodiment enable even the use of a
single base station for the location purposes. The timing advance
(TA) measurement may be also be used together with an angle of
arrival (AOA) measurement from only one BTS site. In this case the
location may be determined to the intersection of AOA line and a
timing advance (TA) circle. A possibility is to use the timing
advance information together with a sector antenna referred to
above. Other known methods, such as observed time difference (OTD)
and time difference of arrival (TDOA) may also be used together
with the timing advance information.
As explained earlier in this specification, a timing advance
information parameter or value is used in time division multiple
access (TDMA) networks, such as the GSM, in order to be able to
separate the time slots in a frame from each other. The timing
advance information is required since propagation of radio waves
between the transmitting and receiving stations causes a delay in
the transmission between the stations. In brief, the timing advance
(TA) information indicates how much earlier the MS should transmit
towards the BTS so that the transmission would be received by the
BTS according to the frame structures used by said receiving BTS.
During a dedicated connection the BTS continuously determines the
appropriate TA value and signals the TA value to the MS. In order
to address the delay, and more precisely, to avoid any overlapping
time slots, it is therefore necessary to determine the time delay
between the mobile station and the serving base transceiver station
(BTS) in order to be able to generate the timing advance (TA)
information describing the offset between the timings used by the
respective stations. The TA parameter may be achieved by defining
the timing of a component of a received signal burst and comparing
the timing of that component to the timing of the same component in
a training sequence.
FIG. 3 shows an impulse response pattern for a received signal
burst which relates to one possibility for determining the timing
based on impulse responses. The impulse response can be obtained by
correlating a known bit pattern in a burst (a training sequence)
with the received signal. The impulse response shown by FIG. 3 is a
curve with the time on X-axis (components taps) and intensity of
the signal on the Y-axis. The first four or five components (taps
on the left) show substantially low or almost nonexistent response
or correlation with the training sequence. After these the response
starts to increase, and the two highest components indicate of an
almost full response.
FIG. 3 shows arrows pointing to three different types of components
(taps) of the response: a first component, a mass center component
and reflected components. In the normal operation of a
communication system it is important that the timings of the
transmitting and receiving stations are adjusted so that a maximum
energy will be received at the receiving station. The timing for
the actual transmission has been conventionally determined from the
mass center of the impulse response, as this has been found to
provide appropriate energy levels, although the adjustment of the
timing for transmission and/or receiving purposes may also be based
on some other approach, such as amplitude or power of the received
signal.
However, a timing providing a best energy may not be the best
timing for use in distance estimation procedures. As can be seen
from the FIG. 3, the mass center can be positioned relatively far
on the right on the X-axis, as it is a weighted value considering
the entire burst taking also all reflected components into account.
In the event of a multipath propagation the impulse response curve
becomes even wider as the reflected signals make the impulse
response asymmetric by moving the intensity to the right in FIG. 3.
In other words, the mass center components do not take the LOS and
NLOS conditions into account. For the purposes of location it
would, however, be advantageous to use a component which indicates
the shortest path between the mobile station and the base station.
This component will in the following be referred to as the first
component.
In brief, the first component is the first component of the
arriving signal that meets a predefined condition or several
conditions. According to one possibility the first component can be
defined as a first arriving component on the rising edge of the
impulse response curve. The first component may also be a first
component at the mass center of a first peak. The predefined
condition is preferably set such that the first component of the
received signal burst is the first arriving component that can be
detected in a sufficiently reliable manner. The first reliably
detected component arrives as a first recognizable component of the
transmission, and thus it must have traveled the shortest path from
the transmitter. In an ideal occasion the first component
represents a line of sight (LOS) situation without being distorted
by any reflections.
In FIG. 3 the condition for the first component is defined by a
threshold level Th. The threshold level is defined such that the
first component exceeding it is the first component giving reliable
enough results for the timing measurements for the location
purposes. It is also possible to use some other conditions for
defining the first component of the received signal burst. These
include, without limiting to these, a tap in the first peak with
maximum height (in case the response includes several peaks), the
first tap with height more than a predefined portion of the height
of the highest tap (e.g. 40%), the first tap in the first peak with
height more than a predefined portion of the height of the first
peak (e.g. 40%), a tap corresponding to the maximum derivative of
the first (main) rising slope (the fastest rise), the first tap
with the height exceeding the signal to noise ration by a
predefined amount (e.g. 1.5 times S/N) and so on. If an impulse
response has more than one peak, the first peak can be found, for
example, by sliding a suitable window over taps, and monitoring the
total sum of the heights of the taps. When this sum has the first
maximum, the first component (tap) is found. Another way is to
trace the derivative of tap heights. The first crossing through
zero indicates the first peak.
The following embodiments discuss in more detail the possibilities
to improve the accuracy of distance estimations based on the timing
information. In the following examples it is supposed that timings
are determined using impulse responses. However, it is to be
appreciated that also other solutions for defining the timing of
the components of a signal can be used when implementing the
invention. Examples of such alternatives include monitoring of
signal amplitudes or power levels.
The block diagrams of FIGS. 4A to 4C illustrates some possibilities
for the embodiments of a base station and a mobile station pair. In
these embodiments it is assumed that the operation is initiated by
the base station 4. It is noted that the first transmission in a
operation cycle in accordance with the embodiments can also be
transmitted by the mobile station 7.
In the first embodiment disclosed by FIG. 4A a base station BTS 4
transmits signal bursts by its internal timing structure in a per
se known manner. The signal may include an indication that a timing
information is required for location purposes. A mobile station 7
is arranged to receive the signal bursts from the BTS and to adjust
its internal reception timing 21 according to the received bursts
in a conventional manner in order to optimize the timing for the
communication. The adjusting is accomplished by a processing unit
20. In our example this can be done using the mass center of the
impulse response of FIG. 3.
In addition to determining the mass center and the timing of the
mass center, a first component of the signal that meets a
predefined condition is detected at the receiving mobile station 7.
After detection, a timing 22 of the first arriving signal component
is determined by the unit 20 for use in determining the distance
between the stations. In the impulse response the first arriving
signal component can be, for example, first one of the taps in the
rising (left) edge exceeding threshold value Th of FIG. 3. It may
also be the mass center of the first peak, if the first peak can be
separated. In most cases the first component is more relevant for
the location purposes than the mass center or any later component,
since it represents the signal coming as directly as possible from
the BTS 4 to the MS 7 (shortest path) under Line of Sight (LOS)
conditions, or at least the signal traveling the shortest reflected
path under Non Line of Sight (NLOS) conditions.
According to an embodiment the mobile station 7 responds to the
received signal burst by transmitting a response back to the BTS 4
by using a different timing from that of the internal (reception)
timing 21. In other words, the timing 22 used for responding is
different from the single timing 21 the MS 7 would use in a
conventional mode of operation for receiving and also for
transmitting. The necessary processing for this can be accomplished
at a processor unit 20 of the MS 7 of FIG. 4A. The timing of the
response transmission is based on the timing 22 of the first signal
component in the impulse response instead of the timing of the mass
center component 21. The BTS 4 receives the signals from the MS and
determines the impulse response. The base station 4 uses the taps
of the trailing sequence corresponding the first component instead
of the taps of the mass center to determine a timing advance (TA)
for the connection. Therefore the timing advance (i.e. the
propagation delay) information generated for use in location, e.g.
by the location server 16 of FIG. 1, will be correspondingly based
on the first component of the transmission. The first component is
believed to give more accurate results for the distance estimation
than the mass center component, as the mass center component is
affected by the reflected signals. After the location procedure is
over, it is preferred that the system returns to normal mode of
operation and use the normal timing parameters for further
communication between the base station and the mobile station.
However, this is not necessary in all instances.
It is possible that the mobile station 7 cannot use the different
reception and transmission time bases 21 and 22 of FIG. 4A.
However, it is possible that the MS 7 adjusts its internal time
base 21 as usual, e.g. using the mass center, and only measures the
difference between the first component and the mass center. The
difference can be a time difference between the components, the
difference between the order numbers of the components or the bits
of the burst. The determined difference is subsequently reported to
the communication network to be used as a correction value in the
subsequent timing based distance determinations. The correction
calculations of the timing advance for use in the distance
estimation can then be accomplished at the base station 4 or in any
other appropriate network element. The advantage of this embodiment
is that all transmission will be sent by using the optimized
timing. e.g. the mass center, instead of using the first component
based timing for e.g. the access bursts.
FIG. 4B shows an embodiment in which both the mobile station 7 and
the base station 7 are provided with internal and location timing
functionalities 21 and 22. This enables a determination of the
timing of the first arriving component in both the transmitting
base station 4 and the receiving/transmitting mobile station 7 and
thus a determination of the distance between the station can be
based on the timing information by the base station or the mobile
station or both.
The above described embodiments require some modifications to the
hardware and/or software of the existing mobile stations. Because a
number of "old" mobile stations cannot perform all parts of the
above described functionality, it may be preferred in some
instances that a mobile station capable of performing the above
procedure indicates to the network that it has used the different
reception and transmission time bases 21 and 22. By means of this
the network is enabled to make a decision whether to apply any
corrections to the timing advance values or not. In addition, by
means of this information the network may initiate a routing
procedure in order to deliver the timing information to the
location service for use in the distance estimation
calculations.
Compared to the present systems, the above procedures eliminate a
double error (both in MS and BTS) caused by reflections in the
radio propagation path. This error can be hundreds of meters.
The following will describe embodiments which do not necessarily
require any changes to the existing mobile stations. A BTS 4 of
FIG. 4C transmits in a usual manner by its internal 21 timing
towards a MS 7. The MS 6 receives the first signal burst from the
BTS 4 and adjusts its internal timing at the processing unit 20 in
accordance with the received burst(s) as usual, e.g. using the mass
center of an impulse response between the received signal and a
known bit pattern. The mobile station 7 subsequently transmits a
response back to the base station 4 using the internal timing based
on the determined mass center of the impulse response.
The BTS 4 receives the response signals from the MS 7 and
determines the impulse response. A processing unit 14 of the BTS
determines two separate timings 21 and 22: currently used timing
based on the mass center and the timing based on the first
component. Thereafter the BTS 4 determines the time difference
between these two timings. The BTS measures also the timing advance
(TA) based on the first component. To compensate an error made by
the MS, the BTS subtracts from the measured timing advance (TA) the
determined time difference between first component and mass center
(based on its own measurements). This is based on the assumption
that the radio channel is unchanged between uplink and downlink
bursts and symmetric. In addition, the MS is assumed to use the
mass center of a similar component in a similar manner to the base
station. In practice this means that the effects of multipaths on
the impulse response, and thus the difference between the mass
center and the first component, is similar both in the BTS and in
the MS.
The largest benefit of the timing advance (TA) location method of
FIG. 4C is that also old (unmodified) mobile stations can be
located. A common view has been that the use of old mobile stations
lead to poor accuracy, but the above embodiment enables a good
accuracy also with old mobile stations.
The location information provided by the communication system can
also be used for purposes other than call processing such as for
routing, charging, resources allocation and so on. There are
several possible commercial and non-commercial applications which
may use this location information. These possible applications
include, without limiting to these: different local advertisement
and information distribution schemes e.g. transmission of
information directed to those mobile users only who are currently
within a certain area; area related WWW-pages such as time tables,
local restaurant, shop or hotel guides, maps, local advertisements
and so on; location of those who have called to an emergency
number; and tracking of mobile users by anyone who wishes to
receive this information and is legally entitled to obtain it. An
application requiring precise and real-time location information of
the movement of a mobile station is a mobile station movement
prediction feature that may be utilized, for example, in dynamic
network resource allocation. There are various other possible uses
of the location information and applications which may use the
location information. In general, all applications which need
geographical location of the mobile station may find the location
information provided and/or processed by means of a
telecommunication system useful. The usability of this location
information is increased by improving the accuracy of the location
information provided by the telecommunications system by the above
discussed embodiments.
It should be appreciated that whilst embodiments of the present
invention have been described in relation to mobile stations,
embodiments of the present invention are applicable to any other
suitable type of user equipment.
It is also noted herein that while the above describes exemplifying
embodiments of the invention, there are several variations and
modifications which may be made to the disclosed solution without
departing from the scope of the present invention as defined in the
appended claims.
* * * * *